1. Field of the Invention
The present invention relates to a discharge lamp lighting controller and a discharge lamp socket used for a discharge lamp that emits light by the discharge accompanying gas breakdown, and particularly to those preferably applied to discharge lamps used as headlights of a vehicle such as an automobile.
2. Description of Related Art
Conventionally, halogen lamps, in which halogenated gas is filled between electrodes, are widely used as headlights of a vehicle. The halogen lamps have an advantage of being able to emit higher density luminous flux more effectively than filament lamps. Recently, attempts have been made to use as headlights of a vehicle instead of the halogen lamps discharge lamps such as high-voltage sodium-vapor lamps and metal halide lamps which have been used as streetlights because discharge lamps can emit even higher density luminous flux than the halogen lamps when driven at a fixed amount of power.
FIG. 16 is a block diagram showing a conventional discharge lamp lighting controller and its peripheral circuits disclosed in Japanese patent application laid-open No. 4-26002. In FIG. 16, the reference numeral 1 designates a DC power supply such as a battery mounted on a vehicle; 2 designates a DC-DC converter for multiplying a power supply voltage from the DC power supply 1 to output a high voltage; 3 designates a DC-AC inverter for converting the high voltage to an AC voltage; 4 designates a discharge lamp to which the AC voltage is applied; 5 designates an igniter circuit provided between the DC-AC inverter 3 and discharge lamp 4 for generating a very high voltage by further boosting the high voltage for firing the discharge lamp 4; 6 designates a control circuit for controlling the DC-DC converter 2 to supply the discharge lamp 4 with a fixed amount of power; and 7 designates a headlight switch connected between the DC power supply 1 and the DC-DC converter 2.
Next, the operation of the conventional system will be described.
When the headlight switch 7 is turned on, the power supply voltage is fed to the DC-DC converter 2. The DC-DC converter 2 multiplies it to the high voltage which passes through the DC-AC inverter 3 which is initially at rest. The igniter circuit 5 further boosts the high voltage and applies it to the discharge lamp 4. This causes breakdown between the electrodes of the discharge lamp 4, thus emitting light involved in arc discharges.
Once the breakdown has occurred, the igniter circuit 5 stops and the DC-AC inverter 3 starts its operation so that the discharge lamp 4, which is supplied with high AC power, continues its emission of light. In this steady state, the control circuit 6 regulates the lamp current supplied from the DC-DC converter 2 to the discharge lamp 4 such that it emits light at a fixed amount of power. This is carried out in the expectation that its life will be prolonged.
The conventional discharge lamp lighting controller with such an arrangement has a problem in that a lamp current more than a rated current which was estimated at the design stage can flow through the DC-DC converter 2 and DC-AC inverter 3.
This is because there are sometimes more than estimated variations in the amount of gas filled in the tube of the discharge lamp 4, and in addition the gas can be gradually discharged from the tube. If the lamp voltage of the discharge lamp 4 reduces owing to these factors, a current more than the rated current will flow. Leaving the discharge lamp 4 in this state will cause an excessive current to continue flowing through the discharge lamp lighting controller, and this will cause secondary problems such as impairment of circuit components.
Reversely, if an increasing lamp voltage is applied to the discharge lamp 4 in association with its life, secondary problems can arise such as explosion of the discharge lamp 4 due to the high voltage.